<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        html,body,.parent {
            width: 100%;
        }
        .parent {
            display: flex;
        }
        .left {
            width: 100px;
            background-color: red;
        }
        .right {
            flex: 1;
            background-color: yellow;
            width: 0;
            overflow: hidden;
        }
    </style>
</head>
<body>
    <div class="parent">
        <div class="left">123</div>
        <div class="right">
            <div style="width:2000px">2000px</div>
        </div>
    </div>
</body>
</html>